Arsenic trioxide corrosion inhibitor for absorption refrigeration system

ABSTRACT

EVOLUTION OF HYDROGEN AND CORROSION OF IRON AND COPPER CONSTRUCTION MATERIALS IN A CLOSED, SUBSTANTIALLY ANAEROBIC REFRIGERATION SYSTEM OF AN ABSORPTION REFRIGERATION MACHINE IS SUCCESSFULLY CONTROLLED BY ADDITION OF AN ARSENIC TRIOXIDE INHIBITOR TO THE LITHIUM BROMIDE ABSORBENT SOLUTION. THE ABSORBENT SOLUTION IS MAINTAINED AT A BASICITY OF FROM 0.1 N TO 0.5 N BASED ON A 60% LITHIUM BROMIDE SOLUTION.

Jan. 19, 1971 J MODAHL ETAL 3,555,841

ARSENIC TRIOXIDE CORROSION INHIBITOR FOR ABSORPTION REFRIGERATION SYSTEMFlled July 15, 1968 3000 TIME,HOURSI FIG.2

United States Patent O 3,555,841 ARSENIC TRIOXIDE CORROSION INHIBITORFOR ABSORPTION REFRIGERATION SYSTEM Robert J. Modahl, Galesville, andPaul J. Lynch, La

Crosse, Wis., assignors to The Trane Company, La

Crosse, Wis., a corporation of Wisconsin Filed July 15, 1968, Ser. No.744,913 Int. Cl. F25b /00 US. Cl. 62-114 12 Claims ABSTRACT OF THEDISCLOSURE Evolution of hydrogen and corrosion of iron and copperconstruction materials in a closed, substantially anaerobicrefrigeration system of an absorption refrigeration machine issuccessfully controlled by addition of an arsenic trioxide inhibitor tothe lithium bromide absorbent solution. The absorbent solution ismaintained at a basicity of from 0.1 N to 0.5 N based on a 60% lithiumbromide solution.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to absorption refrigeration systems and, particularly, tocompositions dissolved in an absorption solution to inhibit corrosion ofinternal metal surfaces of the system.

Description of the prior art The working fluids employed in most largecommercial absorption refrigeration machines are concentrated lithiumbromide solutions as the absorbent and water as the refrigerant. Currentsingle effect machine design utilizes low pressure steam or hot water asthe energy source. These machines operate at maximum absorbent solutiontemperatures of about 250 F.

It is desirable, however, to raise the operating temperatures of thelithium bromide-water absorption cycle to achieve higher efficiency.This is most easily accomplished with a double effect or two-stagegenerator design. The temperatures reached in the high temperaturegenerator of a double effect machine can be as high as 400 F. The heatsource for the high temperature generator is usually high temperaturesteam, although another common method is to directly fire with gas.

In the past effective corrosion inhibitors have been found for absorbentsolutions, but these inhibitors were utilized in systems which containedacidic absorbent solutions or in which there was sufiicient free oxygenpresent to affect corrosive characteristics of the constructionmaterials. It is desirable, however, for optimum operation of absorptionrefrigeration systems to operate with alkaline absorbent solutions in ananaerobic, or no research has been conducted under the combined alkalineand substantially anaerobic conditions.

SUMMARY OF THE INVENTION This invention provides a means for inhibitingand controlling corrosion in high temperature, double effect absorptionrefrigeration systems which comprises an absorber, first and secondeffect generators, first and second effect condensers, and anevaporator, said ab- 3,555,841 Patented Jan. 19, 1971 ice sorber,generators, condensers and evaporator forming a closed, substantiallyanaerobic system. The closed system contains a concentrated, aqueouslithium bromide absorbent salt solution which is maintained at a basicnormality of from about 0.1 to about 0.5, preferably from 0.1 to 0.3.The solution further contains an AS203 inhibitor in an amount greaterthan about 200 mg. per liter of solution, preferably from 200 to 2000mg. per liter of solution, and more preferably from 200 to 700milligrams per liter of absorbent solution.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a schematic representation oftwo-stage or double effect absorption refrigeration systems.

FIG. 2 is a graph showing inhibitor consumption rate as a function ofabsorbent solution normality.

DESCRIPTION OF EMBODIMENTS FIG. 1 is a simplified schematicrepresentation of an absorption refrigeration machine which containsmaterials subject to the corrosive effects of a LiBr salt solution. Theprimary construction material is steel. The heat exchange surfaces areusually manufactured from copper or a copper-nickel alloy. Referringparticularly to FIG. 1, absorber section 10 contains a plurality of heatexchange tubes 11. A spray header 15 is located above the absorbersection.

An evaporator section 12 is situated in the same enclosure with absorber10. In the schematic the evaporator includes a spray header 13 todistribute a refrigerant over heat exchange coils 14. The evaporatorsection 12 and absorber section 10 are in open communication with eachother, although in practice baffles are provided to provided to preventliquid refrigerant entrained in the vapor from carrying over into theabsorber section. A recycle means is also usually provided to returnunevaporated refrigerant from below the evaporator coils 14 to sprayheader 13.

A suitable refrigerant, for example, water, is sprayed over coils 12 inthe evaporator section and a suitable absorbent solution, for example,aqueous lithium bromide, is sprayed over tubes 11. The refrigerant isvaporized in the evaporator section and passes into the absorber sectionwhere the refrigerant vapor is absorbed by the absorbent solution.vaporization of the refrigerant extracts heat from a heat exchangefluid, usually water, passing through coils 14. This heat is carried'with the refrigerant vapor to the absorber where it is released to acooling fluid passing through tubes 11. The refrigerant evaporationproduces a cooling effect on the fluid passing through coils 14.

Dilute absorbent solution 16 is circulated by pump 17 through line 18 toand through a =first heat exchanger 20 and a second heat exchanger 21 inwhich the dilute solution is successively heated before it passes to afirst effect generator 22. Steam from a suitable source is introducedthrough tubes 23 to boil the weak solution, thus driving off water vaporand concentrating the solution. The absorbent solution of intermediateconcentration 24 is forced by pump 25 through line 26 into heatexchanger 21 where it gives up a portion of its heat to the dilutesolution. From heat exchanger 21, the solution of intermediateconcentration passes into a second effect generator 30 which is at alower pressure than the first effect generator 22.

The refrigerant vapor from the first effect generator passes throughline 31 into heat exchange tubes 32 where it transfers its heat ofvaporization to the partially concentrated absorbent solution causingthe latter to again boil. Thus the absorbent solution of intermediateconcentration is converted to a concentrated absorbent solution 33 whichis pumped through line 34 into the first heat exchanger 20. From thefirst heat exchanger, where the concentrated solution transfers heat toweak or dilute absorbent solution, it is pumped by pump 35 via line 36back to spray header in the absorber section 10.

When the refrigerant vapor from the first effect generator 22 gives upheat of vaporization in tubes 32 to further concentrate the absorbentsolution, the refrigerant of course condenses. Thus the second effectgenerator also functions as a first effect condenser. The refrigerantvapor produced in the second effect generator along with the refrigerantfrom tubes 32 passes to a second effect condenser 40 in which all of therefrigerant vapor is condensed.

The refrigerant 42 is collected in pan-like member 41 and transferred tospray header 13 in the evaporator 12 via line 43. The cooling fluid issupplied to coils 44 in the second effect condenser through line 45 fromheat exchange tubes 11 in the absorber 10.

The heat exchange fluid in coils 14 which is cooled by evaporation ofrefrigerant is passed through line 19 to a suitable distribution system.A cooling fluid, for example, water, is supplied to the tubes 11 in theabsorber section 10 from a suitable fluid cooling means such as acooling tower. The cooling fluid is returned to the cooling tower orsimilar means from coils 44 of the second effect condenser 40.

A suitable absorbent solution for this refrigeration system is aconcentrated, aqueous, hygroscopic lithium bromide solution. During theabsorption-concentration cycle, lithium bromide concentration can varyfrom 46% to 65% based on the weight of the solution.

Ideally, the above system is completely closed and operates in theabsence of air or oxygen. However, it is contemplated that a purgeapparatus, shown as 28 in FIG. 1, will be provided to remove air andother non-condensibles as is common in absorption machines of this type.Under normal operating conditions a small amount of air can leak intothe system through weld joints or other imperfect fittings. However, theclosed system described above does operate under substantiallyoxygen-free, or substantially anaerobic conditions.

As previously mentioned, the construction materials of the absorptionrefrigeration machine described above are primarily steel and copper.The control of corrosion in this type of two-stage machine iscomplicated by a number of corrosion mechanisms which occursimultaneously. Of major importance is the type of corrosion whichoccurs under anaerobic conditions, that is, in the absence of freeoxygen, where the water of the absorbent solution reacts with the ironand steel parts of the machine. Iron is oxidized by the hydrogen ions ofthe water producing ferrous and ferric ions and hydrogen gas. Thenon-condensible gas tends to accumulate in the lowest pressure area ofthe machine, the absorber, reducing the rate of absorption of water sothat full refrigeration capacity cannot be sustained. Continued metalcorrosion can eventually lead to structural failure. In addition, theiron ions can combine to form solids which can plug spray nozzles andcoat metal surfaces and reduce heat transfer rates.

Another corrosion problem involves the thermogalvanic corrosion ofcopper and copper alloys. The thermogalvanic corrosion mechanisminvolves the following steps: (1) metallic copper exposed to hightemperature lithium bromide solution is oxidized to form soluble cuprousions; (2) the cuprous ions enter the absorbent solution and aredistributed throughout the machine as the solution is circulated; (3)the copper oxidation half- 4 reaction occurring at high temperaturelocations is balanced by a reduction half-reaction involving the platingout of copper ions from solution in low temperature areas.

With respect to the corrosion of iron and steel, it is generallyadvantageous to maintain the solution on the alkaline side by theaddition of hydroxide if necessary. Proper amounts of hydroxyl ion aslithium hydroxide in lithium bromide solutions markedly reduces the rateof iron and steel corrosion. The reduced rate achieved in this manner,however, is still excessive, particularly at the high temperaturesrequired in a double effect system.

As previously discussed, the lithium bromide solution does not functionunder isothermal conditions in an absorption refrigeration machine. Atypical closed-loop thermal cycle approximates temperatures from 100 F.to 350 F. to 225 F. and back to 100 F. Under such conditions, thethermogalvanic effect causes the corrosion rate to be greater than underisothermal conditions.

EXAMPLES The following examples are intended to illustrate the instantinvention, but they are not to be construed as limiting the scope of theinvention.

Example I A series of tests are made in an apparatus comprising twointerconnected steel vessels forming a closed system. A 60% lithiumbromide solution is placed in the system. Steel and %10%/c0pper-nickelalloy samples are inserted in each of the vessels. These metal sampleshave sufficient surface area to solution volume ratio to approximateactual absorption refrigeration machine design. A measured amount ofcorrosion inhibitor is added to the system. The normality is adjusted tothe desired level by addition of lithium hydroxide. The normality isbased on a 60% lithium bromide solution.

The lithium bromide solution is boiled at 350 F. in the high temperaturevessel and, by vapor lift, pumped into the second vessel which ismaintained at 200 F. The solution is then returned to the hightemperature vessel. Residence time in each of the vessels isapproximately equal.

Results of the above procedure after maintaining the system for about672 hours are described in Table I. The inhibitor and normality arevaried in accord with the table. The 90-10/copper-nickel alloy is codedas Cuni.

TABLE I Inhibitor Metal corrosion Hydrogen Conccntrarate, MPY evolvedtion, mg./l. Alkaec./hr. it.

linity Steel at Cuni at of steel Identity Start End normality 350 F. 350F. at 350 F.

None 0.1 0. 4 0.0 1. 6 AS2O3 1,000 2 0.1 0.0 11.3 0. 1 AS203 l, 000 5880.3 0. 1 2. 2 0. 1 SbzOa- 1, 500 431 0.1 0.3 1. 5 0. 1

Example II Another two vessel test is run following the procedure ofExample I except the high-temperature vessel is held at 400 F. Arsenictrioxide and lithium molybdate in hibitors are contrasted. The resultsafter maintaining the test for about 666 hours are shown in Table II.

TABLE II Inhibitor Metal corrosion Hydrogen Concentrarate, MPY evolvedtion, mg./l. Alkacc./hr. it. linity Steel at Cuni at of steel IdentityStart End normality 400 F. 200 F. at 400 F.

AS203. a... 1, 000 660 0.1 0.1 0. 1 0.1 Ll2l\IO0-l- 2,000 0. 1 0.2 0. 0.6

Note that an excessive hydrogen evolution rate persists with lithiummolybdate. The lithium molybdate concentration at the end of the run wasnot analyzed.

Example III TABLE III Inhibitor Metal corrosion Hydrogen Concentrarate,MPY evolved tion,mg./l. Alkacc./hr. It. linity Steel at Cuni at of steelldentlty Start End normality 400 F. 225 F. at 400 F AszOa 1,000 665 0.30.0 0.0 0.1 SbgOa- 1, 500 158 0. 3 0. 0 0. 0 0.1

Comparing the arsenite and antimonite, metal corrosion and hydrogenevolution rate is satisfactory. In contrast, however, the antimonite hasa consumption rate which is unexpectedly much higher than the arsenite.

Example IV The procedure of Example III is repeated. The hightemperature vessel is boiled at 350 F., the intermediate temperaturevessel is held at 225 F., and the low temperature vessel is held at 100F. Steel samples only are placed in the high temperature vessl, and90l0/coppernickel samples are placed in the intermediate and lowtemperature vessels. Three runs are made using the AS203 inhibitor. Thenormality of the solution in each run is 0.1, 0.3 and 1.0, respectively.

FIG. 2 illustrates the effect of alkalinity on the performance ofarsenite or arsenic trioxide inhibitor. The inhibitor is consumed morerapidly at first in 0.1 N solution than at higher alkalinities. Unlikethe latter, however, the slope of the 0.1 N curve decreases in timeindicating it to be a preferred alkalinity for the long term. Theelapsed times corresponding to a and b represent the total hours ofoperation of 1.0 and 0.3 N solutions before inhibitor concentrationbecomes depressed to the point that hydrogen begins evolving at anexcessive rate. The inhibitor is consumed at a faster rate and hydrogenevolution begins at a higher inhibitor concentration in 1.0 N than in0.3 N solution. Furthermore, addition of 500 mg. per liter of AS203 toeach of these systems after hydrogen evolution had begun resulted incessation in the 0.3 N solution but not in the 1.0 N solution.

GENERAL DESCRIPTION Inhibitors or other additives generally have a lowsolubility in a concentrated aqueous lithium bromide solution. Corrosioninhibition theory states that a minimum concentration of inhibitor isrequired to effect anodic inhibition or passivation. It is believed thatmany of the other oxyanions tested and found ineffecitve were notsufliciently soluble. Those materials affording initial protection, suchas arsenite, are consumed with time while sustaining the protection.Therefore, a reserve is required. The maximum solubility of theseinhibitors in concen trated lithium bromide is less than one gram perliter. Therefore, a long term reserve must either be present in aninsoluble form or be provided by periodic make-up charges proportionalto the consumption rate. The presence of an insoluble excess can beundesirable because it tends to plug spray nozzles causing operatingproblems. It was determined that successful operation of an absorptionmachine can be achieved at arsenite concentrations up to 2000 mg. perliter of absorbent solution. Below a concentration of 200 mg. per literexcessive hydrogen evolution occurs. Optimum operating range is between200 mg. per liter and 700 mg. per liter of absorbent solution, thelatter figure being near the solubility limit of the arsenite in aconcentrated lithium bromide solution.

The preferred operating range of solution normality is from 0.1 N to 0.3N although successful operation is achievable at a solution normality upto 0.5 N. The normality is measured relative to a 60% lithium bromidesolution.

From the foregoing disclosure utility for the instant inhibitorcomposition for use in an absorption refrigeration machine is apparent.

The invention described in this specification is to be limited only bythe appended claims.

What is claimed is:

1. A two-stage absorption refrigeration machine comprising an absorber,a first effect and'a second effect generator, a first effect and secondeffect condenser, an evaporator, an absorbent solution in said machinein contact with construction metal subject to corrosion, means tocirculate said absorbent solution through said machine in contact withsaid metal, said machine being a closed, substantially anaerobic system,said absorbent solution including an aqueous, concentrated lithiumbromide salt solution, said solution having a basic normality of fromabout 0.1 to about 0.5 and further including As O inhibitor.

2. The machine of claim 1 wherein the said metal comprises steel andcopper.

3. The machine of claim 2 wherein hydroxyl ions are provided by lithiumhydroxide.

4. The machine of claim 3 wherein As O is present in an amount greaterthan 200 mg. per liter of absorbent solution.

5. The machine of claim 4 wherein the normality of the said absorbentsolution is from about 0.1 to about 0.3.

6. The machine of claim 5 wherein the AS203 is present in the absorbentsolution in an amount from about 200 mg. per liter to about 2000 mg. perliter of absorbent solution.

7. The machine of claim 6 wherein the As O is present in the solution inan amount from about 200 mg. per liter to about 700 mg. per liter ofsolution.

8. A method of inhibiting corrosion and hydrogen evolution in atwo-stage absorption refrigeration machine, said machine having aclosed, substantially anaerobic systern in which a concentrated, aqueouslithium bromide absorbent solution is circulated and in which systemsaid absorbent solution contacts metals subject to corrosion,comprising:

(a) maintaining the normality of said absorbent solution in the range offrom about 0.1 to about 0.5 by addition of hydroxyl ions,

(b) adding sufiicient AS203 to said solution to maintain the AS203concentration above 200 mg. per liter of absorbent solution.

9. The method of claim 8 wherein the normality is maintained by addinglithium hydroxide.

10. The method of claim 9 wherein the normality of said lithium bromidesolution is maintained in the range of from about 0.1 to about 0.3.

7 8 11. The method of claim 10 wherein the concentra- 2,580,983 1/ 1952Widell 62--112X tion of AS203 is maintained between about 200 mg. per2,755,635 7/1956 Bourne 62-485X liter to about 2000 mg. per liter ofsolution. 3,266,266 8/ 1966 Reid, Jr. 62- 497X 12. The method of claim-11 wherein the AS203 concentration is maintained between about 200 mg.per liter 5 I A J. WYE, Primary Examiner to about 700 mg. per liter ofsolution.

US. Cl. X.R.

References Cited 62 85 112 474 476- 165 104 52 67 UNITED STATES PATENTS1,719,762 7/1929 Gollmar 62114X 10

